Determining the geoposition of a movable or moving platform is a centuries-old problem. The geoposition of a vehicle or observer can be defined as the vector from the observer's vehicle to the Earth in Earth-Centered-Earth-Fixed coordinate frame. This geoposition problem was solved in the past by the use of the slight reduction method, which involves incorporation of star altitudes from the horizon to compute lines of positions, the intersections of which define the estimate of the position of the vehicle or observer. The altitudes of the stars relative to the horizon can be determined by the use of a sextant. This sight reduction method requires either visual measurement of the horizon or inertial measurement of the local gravity vector as a benchmark for all star observations. The visual observation of the horizon is subject to error due to obstructions, haze and vehicle motion, and determination of the local gravity vector is rendered inaccurate due to inability to distinguish local gravity from acceleration.
Position of a movable platform can also be determined by dead reckoning, where measurements of angular rotation rates and translational accelerations, using gyroscopes and accelerometers, are integrated into the calculated position. The dead reckoning technique is, generally speaking, only useful for short periods of navigation, as the measurements of rotation rates and acceleration are subject to unavoidable drift errors. The drift errors cumulate with time.
Position of a movable vehicle has also been calculated by triangulating from three or four Earth satellite position measurements. This method has the disadvantage that it is dependent upon absolute knowledge of the positions of the satellites. The orbits of the satellites are affected or perturbed by factors including atmospheric drag and solar winds. While the orbital position can be corrected by periodic position update information derived from ground-based observations, that information is liable to be old at the time that it is applied, which reduces its usefulness for accurate geoposition determination. This, in turn, may result in an error of many kilometers in the geoposition so determined.
A common method for geolocation of a moving vehicle is by use of the Global Positioning System (GPS). The GPS is very effective, but cannot be relied upon because of the danger of outage, such as might be caused by jamming or outright destruction. Consequently, it is desirable to have a technique for accurate geoposition determination without the use of GPS.
United States patent application 2009/0177398 A1, published Jul. 9, 2009 in the name of Belenkii et al. describes an aircraft navigation system with an inertial measurement unit (IMU) and an optical sensor which periodically updates the IMU to compensate for gyroscope and accelerometer drift. As with the case of triangulation, the possibility of inaccurate satellite ephemerides may lead to limited geoposition accuracy.
Improved and/or alternative geoposition determination techniques are desired.